Diseases that result from the combined or interactive effects of two or more genetic variants, with or without environmental factors, are called complex diseases and include cancer, coronary artery disease, diabetes, stroke, and chronic obstructive pulmonary disease (COPD). Although combining non-genetic risk factors to determine a risk level of outcome has been in applied to coronary artery disease, (by combining individual factors such as blood pressure, gender, fasting cholesterol, and smoking status), there are no such methods in combining the effects of multiple genetic factors with non-genetic factors. There is a growing realization that the complex diseases, for which examples are given above, may result from the combined effects of common genetic variants or polymorphisms rather than mutations which are rare (believed to be present in less than 1% of the general population). Moreover, these relatively common polymorphisms can confer either susceptibility and/or protective effects on the development of these diseases. In addition, the likelihood that these polymorphisms are actually expressed (termed penetrance) as a disease or clinical manifestation requires a quantum of environmental exposure before such a genetic tendency can be clinically detected.